The present invention relates to a contactless chip card having an adjustable demodulation unit for demodulating an amplitude-modulated carrier signal and to a method for controlling an adjustable demodulation unit.
Contactless IC (Integrated Circuit) cards, so-called contactless chip cards or smart cards, are usually operated in an electromagnetic field provided by a read/write unit.
The read/write unit usually provides a standardized carrier frequency of 13.56 MHz in the electromagnetic field. On the one hand, the carrier frequency provided uses induction to supply the contactless chip card with energy needed for the operation of the latter and, on the other hand, uses the execution of a communication protocol to ensure communication between the read/write unit and the contactless chip card.
If the contactless chip card is in the electromagnetic field provided by the read/write unit, it communicates with the read/write unit by amplitude-modulating at least one parameter of the carrier frequency. In this case, the degree of modulation of an amplitude-modulated carrier frequency is usually somewhere in the range between 6% and 30% and is then also referred to as the modulation index. On account of the wide range of possible modulation indices, problems of compatibility with conventional demodulation circuits may arise. Increasing demands on demodulation units usually result in demodulation circuits which are becoming more and more complex.